This invention relates to an optical modulation device utilizing the electrooptic effect.
In recent years there have been increased cases where optical signals are handled which are prepared by modulating the light beam such as a laser beam with a high frequency signal. For example, such optical signals have been and are utilized in recording on video discs, PCM (which is abbreviated for "pulse code modulation") audio discs etc. Optical modulators for modulating the light beam with a high frequency signal are sorted into what utilizes the electrooptic effect and is called hereinafter an "EO type optical modulator" and what utilizes the diffraction of light by an ultrasonic wave and is called hereinafter an "AO type optical modulator". The EO type optical modulator has a wide operating band and a power resistance but has been disadvangeous in that it has a very large drift to make its use difficult. On the other hand, the AO type optical modulator is small in drift and easily handled but has the disadvantages that it has a narrow operating band and lacks in power resistance.
For recording ultra-high density information on video discs or the like, the wide operating band is generally indispensable. For this reason, the EO type optical modulator has been previously, in many cases employed although it is very large in drift and difficult to be used.
Conventional optical modulation devices employing the EO type optical modulator have been usually constructed so that the differential amplifier produces a difference signal between a reference signal representing a proper bias level and a comparison signal representing the modulated state and supplies the difference signal to the optical modulator thereby to control a drift of a bias level due to, for example, a variation in temperature.
However, those conventional devices have been required to include an expensive beam splitter and an expensive optical detector at each of the entrance and exit of the optical modulator. This has resulted in the disadvantages that the overall cost is increased and time and labor is required to set the optical axis of the optical modulator to the optical axes of its components. Also in such conventional devices, a bias supplied to the optical modulator has been controlled so that a duty ratio is of 50% at the level of the mean value of a modulated intensity-of-light signal. This means that conventional optical modulation devices have been subjected to such a limitation that the duty ratio be of 50% at the level of the mean value of the modulated intensity-of-high signals.
Accordingly, it is an object of the present invention to provide a new and improved optical modulation device capable of controlling a bias applied to an optical modulator involved by a single optical detector alone.
It is another object of the present invention to provide a new and improved optical modulation device for modulating a light beam at a proper bias even though a duty ratio has any value at the level of the mean value of a modulated intensity-of-light signal.